I have very little experience with digital painting, but a little more with traditional oils. Hopefully the interwebs doesnt screw too much with the color in this sketch either. Anyway, I've found that the quickest way to get a quasi-realistic look to your digital paint without rendering it is to:

1)Paint the highlight and the reflected light using a variation of the same color. Not only does this keep the colors in the same family, but it's faster! Notice below that the reflected light is almost exactly the same color ( with few exceptions) as the highlight.
2) Also, the terminator can become too flat very quickly. This is where the light ends and the shadow side of an object begin. As a result, the tendency is to over-render (i.e. gradient transition) this region. Instead, use a definate color change. This will help you get a better idea if the color you chose for the terminator "feels" right. Also, dont be afraid to use that undo key!! The easiest way Ive found is to choose a color for the terminator and paint it in one quick, wavy stroke. U should be able to tell immediately if its off. Simply hit the undo key and try a different color. Hope that helps.

--Pedro